The present invention relates to uninterruptible power supply (UPS) apparatus and methods and, more particularly, to parallel redundant UPS apparatus and methods.
A variety of different techniques have been used to improve reliability of uninterruptible power supply systems. The techniques include standby redundant, serial redundant, and parallel redundant approaches. A typical standby redundant UPS configuration includes one or more UPS units operating on a stand-by basis, with no load or only a partial load, which can immediately back up a faulty UPS unit by a transfer of the load. A typical serial redundant arrangement involves first and second UPSs connected in a serial fashion wherein, in a first mode of operation, the first UPS is bypassed while the second UPS is serving the load and, in a second mode of operation, the second UPS is bypassed while the first UPS serves the load, such that the first and second UPSs may serve as standby backups for one another.
In a typical parallel redundant arrangement, multiple uninterruptible power supplies (UPSs) are coupled in parallel to a load to provide redundancy and, often, increased load capability. Parallel redundant arrangements of AC power supplies (e.g., UPSs) are described, for example, in U.S. Pat. No. 5,745,357 to Tassitino, Jr. et al., U.S. Pat. No. 6,549,440 to Tassitino, Jr. et al., U.S. Pat. No. 6,803,679 to Luo et al., U.S. Pat. No. 6,118,680 to Wallace et al., U.S. Pat. No. 4,104,539 to Hase, United States Patent Publication No. 2005/0162792 to Wang et al., and United States Patent Publication No. 2005/0073783 to Luo et al.